<template>
    <div>
        <ul class="list">
            <li class="item" v-for="item of letters" 
            :key="item"
            :ref="item"
            @click="handleLetterClick"
            @touchstart.prevent="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
            >{{item}}</li>
        </ul>
    </div>
</template>
<script>
export default {
    name:"CityAlphabet",
    props:{
        cities:Object
    },
    computed: {
        letters () {
            const letters = []
            for (let i in this.cities) {
                letters.push(i)
            }
            return letters
        }
    },
    data () {
        return {
            touchStatus:false,
            startY:0,
            timer: null
        }
    },
    updated() {
        this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
            this.$emit('change',e.target.innerText)
        },
    handleTouchStart(){
        this.touchStatus = true
    },
    handleTouchMove(e){
        if (this.touchStatus) {
            if(this.timer) {
                clearTimeout(this.timer)
            }
            this.timer = setTimeout(() => {
                const touchY = e.touches[0].clientY - 79
                const index  = Math.floor((touchY - this.startY) / 20)
                if (index >= 0 && index < this.letters.length) {
                this.$emit('change',this.letters[index])
              }
            }, 10);
        }
    },
    handleTouchEnd(){
        this.touchStatus = false
    }
  }
}
</script>
<style lang="stylus" scoped>
@import '~styles/varibles.styl'
.list
  display :flex
  flex-direction :column
  justify-content :center
  position :absolute
  top :1.58rem
  right :0
  bottom 0
  width :.4rem
  .item
    text-align :center
    line-height :.4rem
    color :$bgColor
</style>